1. Field of the Invention
This invention relates to a zoom lens, and in particular to a zoom lens which is suitable for a microfilm reader, a printer, micro-photography and the like and in which a magnification change is effected with the distance between an object and the image thereof being maintained at a predetermined finite distance.
2. Related Background Art
In the projection optical systems of microfilm readers, printers, copying machines, etc., there have heretofore been proposed various zoom lenses designed to effect a magnification change while the distance between an object and the image thereof is maintained at a predetermined finite distance and continuously obtain a projection magnification.
Such zoom lenses, as compared with those of the turret type in which a plurality of fixed-focus lenses are rotated, have a feature that the projection magnification can be obtained continuously and quickly and moreover easily. For example, U.S. Pat. No. 4,474,436 (Japanese Laid-Open (Kokai) Patent Application No. 73715/1982) and Japanese Laid-Open (Kokai) Patent Application No. 59306/1986 disclose so-called two-group type zoom lenses which have, in succession from the object side, i.e., the enlarging side, a first lens group of negative refractive power and a second lens group of positive refractive power and in which a magnification change is effected with the distance between the object and the image thereof being maintained at a predetermined finite distance and with the spacing between the two lens groups being varied.
However, in the former, the photographing magnification has been 7 to 15 times and F-number has been F4.6-3.4, and in the latter, the photographing magnification has been 6 to 10 times and F-number has been F4.8-4.5, and both of them have been low in photographing magnification and dark in F-number.